The case centered on Allen Ryan Alleyne, who was convicted of using a firearm for a crime after he robbed a store manager who was driving cash to a bank at the end of his shift — a crime that carried a 5-year minimum sentence.

The jury did not find he had "brandished" the weapon, which carried a 7-year mandatory minimum sentence. A judge said Alleyne could still get the 7-year minimum, finding a jury didn't need to find him guilty of "brandishing" the gun and that a judge could make that finding. A federal appeals court affirmed that ruling.

While it's unusual for Thomas to side with the liberals, he does have a history of favoring defendants' right to a jury trial. In 1998, Thomas ruled that people or companies sued for copyright infringement have a right to a jury trial when the plaintiff wants statutory damages.

It's also not entirely surprising that a conservative justice would side with liberals in a criminal case. Unlike cases involving like affirmative action and gay marriage, criminal cases can be less politicized for the court.